1. Field of Invention
The present invention is generally related to hydraulic machines, and specifically relates to a hydraulic pumping device of a plastic and metal construction utilizing virtually lubricant free fluid.
2. Description of the Prior Art
Hydraulic piston machines, such as the one described in PCT/DK94/00001, comprise at least one piston and cylinder where the piston movement is controlled by an incline plate, otherwise known as a control surface. Each piston is placed into a cylinder contained within a cylinder drum and lies on the control surface via the intermediary of a slider shoe. The piston and slider shoe unit operates either axially or radially. In both cases, however, the movement of the piston is restricted by way of the control surface, that is, as the piston moves, the angular position of the slider shoe changes. Hence, by altering the inclination of the control surface, the stroke volume of the piston is changed.
The piston and slider shoe act as a unit and form a first contact surface. The slider shoe and control surface form a second contact surface. During the operation of the hydraulic machine, through movement of respective parts, friction occurs at both the first and second contact surfaces. To reduce friction and wear and tear on the parts, contact surfaces are typically lubricated with hydraulic fluid. The choice of hydraulic fluids, however, is restricted to those liquids which provide satisfactory lubrication. Moreover, many hydraulic fluids such as synthetic oils are disfavored in the ever expanding debate on environmental protection, and replacing these oils is possible only to a limited extent since satisfactory lubrication is not ensured in all cases.
Hydraulic machines known to those skilled in the art utilize a number of different techniques to fix the slider shoe to the piston and reduce or eliminate the need for hydraulic fluid. Each technique attempts to ensure that the movement between the slider shoe and piston is maximized. For example, it is well known in the art to join the slider shoe and piston to one another by way of a ball and socket joint. U.S. Pat. No. 3,183,848 illustrates a pump having an axially operating piston in which the slider shoe is made of nylon and is secured to the ball of the ball and socket joint by means of a metal clip.
More recently, to eliminate the need for hydraulic lubricating fluid, a piston and slider shoe unit has been developed with a friction reducing layer. This layer comprises a plastic material mixed with fibers for use between the slider shoe and the control surface as shown in JP 2-125 979A. Although capable of reducing friction between these moving parts, fixing a separate layer to the slider shoe is relatively complicated. The contact surface must first be roughened or grooved so that the friction reducing layer may be secured to the surface with an adhesive. And because the adhesive bond is stressed with primarily shearing forces, there is a risk that the bond will not hold, and the plastic layer will detach and damage the machine. Stability and strength of the piston and slider shoe unit is also compromised. Moreover, there is a risk that the too much friction will develop between the piston and slider shoe which ultimately leads to the joint seizing or binding, again, risking damage to the hydraulic machine.
Hence, use of the friction reducing layer has been expanded to more than one contact surface as demonstrated in PCT Application No. PCT/DK93/00443. While safeguarding two or more contact surfaces, this friction reducing layer has several disadvantages. First, like the single friction reducing layer, this plastic layer is a functionally separate machine element, and may be displaced by forces on either component. Second, hydraulic fluid under pressure may penetrate between the friction reducing layer and the slider shoe, destroying the cohesion between the contact surfaces and the friction reducing layer. Third, although essentially replacing the lubricating function of hydraulic fluid, if the materials of the friction reducing layer are not correctly matched to the material of the moving part, the coefficients of friction between the reducing layers will not be comparable with the coefficients of friction of a liquid lubricated contact surface. Lastly, to construct a three dimensional friction reducing layer is complicated. While it is preferable for the friction reducing layer to be in the form of a molded part which increases dimensional tolerances, any dimensional inconsistencies must be back-filled during molding. Moreover, surface structures such channels or pockets in the friction reducing layer are preferred for molded parts. Such structures serve to relieve the hydrostatic pressure between the slider shoe and the control surface, and equalize forces in order to stabilized the slider shoe. However, this also complicates the molding process.
A need exists therefore, for a simpler, more reliable and cost efficient hydraulic pumping device that reduces friction between contact surfaces to an operative level without the need for lubricating fluid.